The present invention relates to a new and improved device for monitoring the yarn travel at a textile machine. The device comprises a yarn scanning structure including a flat or plate-like cantilever member which can be excited into oscillation by the traveling yarn, and mechano-electrical transducing means, e.g., a piezoelectric transducer element responsive to such oscillation.
Generally, such a device serves for stopping the textile machine when the yarn breaks or ceases to travel on its predetermined path in the machine.
U.S. Pat. No. 3,467,149 and Swiss Pat. No. 441,172 disclose electronic devices for surveying the presence of weft thread in a shuttle loom which comprise a piezoelectric signal generator arranged in the shuttle. As shown in detail in said United States patent, the shuttle is formed with a hollow central chamber, within which is mounted a weft bobbin. The weft thread is played out from the front end of the bobbin and passes over the signal generator near the front of the shuttle; and then it leaves the shuttle via an output guide. The signal generator itself comprises a base member mounted in the shuttle between rubber bearings, an elongated piezoelectric crystal and a wire-like L-shaped thread feeler element both of which are mounted at the base member. The elongated piezoelectric crystal is supported at each end thereof by means of mounting elements such as to extend horizontally above the base member and transversely to the shuttle. The thread feeler element is mounted at one end thereof on the base member and bends over to extend above and in parallel relationship to the elongated piezoelectric crystal. A vibration coupling member interconnects the thread feeler element and the piezoelectric crystal at a point midway between the two mounting elements. The weft thread presses slightly downwards upon the wire-like thread feeler element traversing along the free end thereof on its way from the free end of the weft bobbin to the output guide of the shuttle.
In piezoelectric signal generators of the prior art elastic members or elements are generally provided for mounting the piezoelectric crystal and coupling same to the thread feeler element. Such a coupling member acts as a vibration transferring as well as a vibration damping means when the signal is generated. Elastic mounting elements generally cause damping or attenuation of the signal as well as suppression of noise and unwanted vibration and shock which might damage the piezoelectric crystal. A certain amount of damping may be advantageous in order to let the yarn travel signal decay rapidly when the yarn breaks. However, when the vibratory transducer system has a relatively high fundamental frequency and high inherent attenuation, additional elastic damping elements might induce undesired loss to the yarn travel signal.
Moreover, a piezoelectric crystal or other vibratory transducer element supported or clamped at each end and working in flexural vibration mode has a high flexural stiffness and thus requires a high force for being excited into oscillation. Additionally, such a transducer element has a high fundamental frequency compared with a unilaterally clamped or cantilever transducer element of equal structure and dimensions. As a consequence, a piezoelectric transducer element supported or clamped at each end furnishes a relatively low signal when excited directly or indirectly by a traveling yarn.
Now one might seek to enhance the mechano-electrical sensitivity or response to mechanical vibration of such a piezoelectric or other vibratory transducer system by constructive measures. However, such measures generally cause not only the desired signal, i.e., the yarn travel signal, but also the undesired noise signals to be increased. In textile factories such noise signals may be produced by ambient sound transmitted by air or by vibrations produced in the textile machine and conducted through solid elements to the transducer system.
Air transmitted sound may be generated by auxiliary devices mounted at the textile machine itself, by fans, compressed air tools or other machines, whereas the vibrations transmitted through solid elements mainly come from the textile machine itself at which the vibratory transducer system is mounted. A serious problem is the sound produced by compressed air tools generally used for cleaning the textile machines, which has a broad frequency spectrum and thus is difficult to neutralize.